If the information contained in the image is important to the meaning of your page (i.e. some important content would be lost if the image was removed) , then you must provide a longer description than the "alt" attribute can reasonably display. The "longdesc" attribute was created for this reason.

<IMG SRC="graph1.gif" LONGDESC="graph1.htm" ALT="3-d sales chart.">

Future browsers or other agents will provide an optional a link to the description file called "graph1.htm".